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1. Installability of a Multiline Ring Anchor System in a Seabed under Severe Environmental Conditions

   Lee, J.; Hong, J.; Aubeny, C.; Arwade, S.; DeGroot, D.; Beemer, R.; Balakrishnan, K.; Nam, Y. (January 2021, IEEE Oceans 2021)

   The trend of offshore wind energy in deeper water that is expected to shift from fixed to floating platforms requires a cost-effective anchor solution for floating offshore wind turbines (FOWTs). Multiline ring anchor (MRA) has been developed as a cost-effective solution for FOWTs due to its capability of anchoring multiple mooring lines, its high efficiency, and its availability to a wide range of soils and loading conditions. While previous preliminary studies on the anchor performance provide useful insights on how the potential advantages of the MRA can improve load capacity, these studies are limited to focusing on optimizing the anchor design in certain soil and loading conditions. By contrast, the MRA will be installed in seabeds under more complex conditions that depend on geological location, water depth of at-place, and environmental conditions, of which wind, current, and wave are major components. These may result in additional substantial extra capital costs, delays in the projects, and safety issues, when the complex conditions are not properly considered. Specifically, the installation time and expenses of the offshore anchor are very susceptible to anchor types, installation methods, and environmental conditions. For this reason, this paper compares two existing offshore anchor installation methods and different anchor types on the basis of their performance under the same severe environmental condition. In evaluating the installability of the MRA, this paper conducts a comparative scenario study. The results show that the anchor installations and anchor handling vessel (AHV) operations are sensitive to weather conditions and AHV sizes. In view of total weather standby, the results show that anchor types or installation methods have little effect on it due to their relatively shorter duration than other work sequences. However, the MRA can benefit in substantially reducing transport time and costs due to its compact size. The MRA can be more efficient and cost-effective than other alternatives under complex and severe weather conditions. 

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2. Uplift resistance of a multiline ring anchor system in soft clay to extreme conditions

   https://doi.org/10.1061/9780784483688.041  

   Lee, J.; Hong, J; Aubeny, C.; Arwade, S.; DeGroot, D.; Martinez, A.; Beemer, R. (November 2021, Proceedings, Geo-Extreme 2021, GSP 328)

   A multiline ring anchor (MRA) system has been developed as a cost-effective alternative for securing arrays of floating offshore wind turbines (FOWTs) to the seabed. Multiline attachments can improve the economically competitiveness of FOWTs by reducing the capital cost of the support system for the floating structures. FOWTs can be subjected to severe wind and wave conditions resulting in extreme loads to the anchor system. Thus, the reliable design of the anchor system requires proper determination of the extreme mooring line loads acting on the anchor needed to secure FOWTs to the seabed. Previous studies showed the MRA in soft clay has clear advantages over existing anchors under the extreme horizontal loading conditions imposed by catenary moorings; however, its performance relative to conventional anchors under extreme vertical loading imposed by taut mooring systems requires further investigation. This study presents predictions of extreme loads on floating structures secured by taut mooring systems and evaluates the potential for developing an economical anchor for resisting these extreme loads. 

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3. Effect of Keying Flaps on a Multiline Ring Anchor in Soft Clay

   https://doi.org/10.1061/9780784483404.023  

   Lee, J; Aubeny, C. (May 2021, International Foundations Congress and Equipment Expo 2021)

   The multiline ring anchor (MRA) was devised as a cost-effective means for securing floating offshore wind turbines (FOWTs) to the seabed. FOWTs occurring in arrays create the possibility for attaching mooring lines from multiple units to a single anchor. Additionally, the deep embedment of the MRA into relatively strong soil permits high load capacity to be achievable with a small and lighter anchor, thereby reducing anchor material, transport, and installation costs. However, since the MRA is shorter than a conventional caisson, features such as wing plates and keying flaps are needed to achieve parity in load capacity with a caisson having a comparable diameter. Preliminary studies show that attaching wing plates to MRA in soft clay is highly effective in enhancing its horizontal load capacity, but only marginally effective in improving vertical load capacity. This motivated the current study investigating the use of keying flaps to further enhance vertical load capacity. Two-dimensional finite element analyses were conducted to understand how keying flaps impact the failure mechanism of the stiffeners and provide reliable evaluations of the uplift resistance of the MRA. The results show that the thickness of the stiffener, flap length, and flap angle can affect the failure mechanism and bearing factors. For the optimal design of the stiffener, a comparative study was carried out to compare the effects of keying flaps and thickness of the stiffener. The studies show that introducing keying flaps can have comparable load capacity with thicker stiffeners and that it can be an economical solution for achieving high vertical load capacity while containing material and fabrication costs. 

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4. Centrifuge Modeling of the Monotonic Capacity of Offshore Ring Anchors in Sand

   https://doi.org/10.37308/DFIJnl.20230915.297  

   Huang, L; Martinez, A; Aubeny, C; DeGroot, D; Arwade, S; Beemer, R (May 2024, DFI Journal The Journal of the Deep Foundations Institute)

   The development of offshore wind technology has become a feasible solution to meet the increasing demands for clean and renewable energy. The United States has a total of 4250GW offshore wind energy potential; however, 65% of it is in deep water zones (Lopez et al., 2022) where wind turbines with fixed foundations typically are economically and technically unfeasible. In those situations, floating turbines supported by subsea anchors are a more competitive solution. Based on previous studies, ring anchors can be more material-efficient than piles and caissons because they require less material. Ring anchors also perform better than drag anchors due to their greater embedment depth. To further understand the behavior of ring anchors in saturated sand, a series of centrifuge load tests were performed at the University of California Davis Center for Geotechnical Modeling (CGM) at an acceleration of 70g. This test series investigated the effect of the anchor embedment depth and loading angle on the monotonic loading behavior. The ring anchor models were embedded in dense saturated sand, and then connected to an actuator using taut steel wire ropes. Sensors were used to measure the line tension, displacement, and inclination. The results indicate that the ring anchors mobilize greater capacities as their embedment depth is increased and when they are loaded at angles close to the horizontal direction, while vertical loading leads to the smallest capacity. The anchor displacement during the tests deviated slightly from the loading direction, showing a horizontal deviation at the earlier stages of the tests and a vertical one after the peak load. Furthermore, soil disturbance induced by the anchor installation was found to have a strong effect on the vertical capacity of the ring anchors. Overall, this study provides valuable information regarding the monotonic loading behavior of ring anchors which can aid in their future field deployment. 

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5. Wind tunnel experiments and model predictions of the performance of a floating offshore wind turbine undergoing pitch motion

   https://doi.org/10.1063/5.0301237  

   Panthi, Keshav; Iungo, Giacomo Valerio (December 2025, Journal of Renewable and Sustainable Energy)

   Floating offshore wind turbines (FOWTs) experience multiple degree-of-freedom (DOF) motion as a result of the non-linear interactions between the aerodynamic and hydrodynamic forces exerted on the turbine rotor and the floating platform, respectively, which create complex dynamics for FOWT operations and, in turn, variability in rotor angular speed and power capture. In this work, wind tunnel experiments are performed with a down-scaled FOWT model installed on top of a robotic emulator that reproduces 4-DOF motions. Rotor rotational speed, ω, and power capture are measured for pitch motions with different amplitudes and frequencies. These experimental data are first analyzed, then used for the validation of a non-linear dynamic analytical model that predicts the variation in ω and power capture by leveraging the aerodynamic quasi-steady assumption, namely, the FOWT power curve measured under static conditions and null pitch angle is used to predict operations under dynamic conditions. The results show that good accuracy is generally achieved with the analytical model. However, dynamic aerodynamic effects occur during pitch motion that can jeopardize the accuracy of the analytical model, especially with increasing ω, motion amplitude, and in correspondence with pitch angles where the inversion of the motion direction occurs. Furthermore, it is found that these dynamic aerodynamic effects can be accurately predicted through a random forest model by providing as input pitch angle, velocity, and acceleration of the incoming wind. Among the different FOWT motion parameters, the pitch angle is found to be the most influential factor for the magnitude of the dynamic aerodynamic effects. 

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